Method and apparatus for froth flotation

ABSTRACT

A process of separating a desired constituent from a mixture of particulate matter including the steps of: conditioning a liquid mixture of particulate matter with a frothing agent to create a pulp; aerating the pulp to generate a float fraction of froth supported on the surface of a non-float fraction of pulp; separating a portion of froth from the float fraction; draining the separated froth; washing the separated froth with a liquid to dislodge particles comprising one or more of non-selectively attached, entrained, and entrapped particles; and recovering the washed froth, is disclosed herein. Also disclosed herein are a froth cleaning apparatus and a froth flotation apparatus for separating a desired constituent from a mixture of particulate matter. The froth cleaning apparatus includes a hood including a lower peripheral edge for interface with the top of a froth flotation cell; a discharge orifice disposed in the hood; a froth support in communication with the discharge outlet for receiving and supporting froth; and a wash sprayer disposed upstream of the discharge orifice. The flotation apparatus includes: a wall defining a flotation cell; an aerator for aerating a mixture of particulate matter to produce froth; a feed opening for introducing a mixture of particulate matter and/or froth into the cell; a discharge orifice in a wall of the cell; a froth support in communication with the discharge outlet for receiving and supporting the froth; and a wash sprayer disposed upstream of the discharge orifice.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to the concentration or beneficiation ofminerals and other particulate matter by froth flotation, and moreparticularly relates to a method and apparatus for concentration orbeneficiation of particulate matter separated from undesired waste bymeans of froth flotation.

2. Brief Description of Related Technology

Commercially valuable substances, such as coal and minerals, arecommonly found in nature mixed with relatively large quantities orprohibitive quantities of unwanted substances. As a consequence, it isusually necessary to beneficiate or clean ores to concentrate a desiredsubstance or, put another way, reduce the content of an unwantedsubstance. Similarly, recycling processes, such as de-inking of paperfibers, involve the separation of a desired substance (paper fibers)from an unwanted substance (ink).

Mixtures of finely-divided product particles and finely-divided wasteparticles can be separated and concentrates obtained therefrom by frothflotation techniques. Generally, froth flotation involves conditioning aliquid, commonly aqueous, pulp (or slurry) of the mixture of product andwaste particles with one or more frothing agents and optional reagents,and aerating the pulp. The conditioned pulp is aerated by introducinginto the pulp a plurality of air bubbles which tend to become attachedto either the product particles or the waste particles, thereby causingthese particles to rise and generate a float fraction of froth on thesurface of a non-float fraction of pulp. The difference in densitybetween air bubbles and water provides buoyancy that preferentiallylifts hydrophobic solid particles to the surface. In known processes,the float fraction overflows or is skimmed from the flotation apparatus.

Froth flotation is often used to separate solids of similar densitiesand sizes, which factors prevent other types of separations based ongravity that might otherwise be employed. It is especially useful forparticle sizes below about 100 μm (about 150 mesh), which are typicallytoo small for gravity separation using jigging and tabling. Thelower-size limit for flotation separation is typically about 35 μm(about 400 mesh). At smaller particle sizes, it becomes difficult totake advantage of surface-property differences to induce selectivehydrophobicity. On the other hand, particles greater than about 200 μm(about 65 mesh) tend to be readily sheared from bubble surfaces bycollision with other particles or vessel walls.

Today, at least 100 different minerals, including almost all of theworld's copper, lead, zinc, nickel, silver, molybdenum, manganese,chromium, cobalt, tungsten, and titanium, are processed using frothflotation. Another major usage of froth flotation is by the coalindustry for desulfurization and the recovery of fine coal, oncediscarded as waste. Since the 1950's, flotation has also been applied inmany non-mineral industries including sewage treatment; waterpurification; paper de-inking; and chemical, plastics, and foodprocessing.

In conventional subaeration cells, the pulp ordinarily is aerated bymeans of a mechanical impeller-type agitator and aerator which extendsdown into the body of pulp and which disperses minute bubbles of airthroughout the body of pulp by vigorous mechanical agitation of thepulp.

In conventional froth-flotation columns, air for aeration is introduceddirectly into a relatively quiescent body of pulp by means of an airdiffuser or sparger which is immersed in or in direct contact with thepulp, or by introduction of pre-aerated water, e.g. from below aflotation compartment.

Generally, subaeration cells have a relatively higher throughput thanfroth-flotation columns, but froth-flotation columns can provide betterseparation between desired and undesired components. As a consequence,when both high throughput and good separation are desired, subaerationcells typically are used in series and froth-flotation columns are usedin parallel. In some cases, the flotation operations are conducted instages wherein the concentrate obtained from the float fraction in onestage can comprise a different substance from the concentrate obtainedfrom the float fraction in another stage.

Typical undesired impurities in coal include pyrite, sulfur, and otherash-forming mineral matter. Pyrite in many U.S. coals occurs in largequantities as fine-grained matter varying in size between 20 microns(μm) and 32 μm. In some coals, such as is available in Illinois, asignificant part of the pyrite is less than 20 μm. To make use of thesetypes of coals more fully, a coal cleaning method capable of processingvery finely ground coal in which most of the pyrite particles have beenliberated must be used. Similarly, reduction in or removal ofash-forming matter can improve marketability and heat content of cleanedcoals, because ash is incombustible and has been linked to poor heatexchange and reduced boiler performance.

In addition, because every coal mine and preparation plant producesfines in the course of extracting and processing coal, failure torecover coal from fines increases the proportion of produced coal thatis discharged into the environment (e.g., into tailing ponds) whichresults not only in a loss of potential revenue but also in anenvironmental impact.

The separation of fine particles by froth flotation techniques presentsparticular obstacles which are only overcome with great difficulty andcost by known techniques, such as use of multiple machines in series orparallel, and known techniques still have limitations in the degree ofseparation which can be achieved.

Thus, it is a continuous goal in the industry to have methods andapparatus which improve the separation of desired particulate matterfrom undesired particle matter.

SUMMARY

One aspect of the disclosure is a method of separating a desiredconstituent from a mixture of particulate matter, including the steps ofaerating a pulp to generate a float fraction of froth supported on thesurface of a non-float fraction of pulp, separating a portion of frothfrom the float fraction, draining the separated froth, washing theseparated froth to dislodge one or more undesired constituents, andrecovering the washed froth.

Another aspect of the disclosure is an apparatus for separating adesired constituent from a mixture of particulate matter, the apparatusincluding at least one wall, preferably walls, defining a flotationcell; an aerator for aerating a mixture of particulate matter to producefroth; a feed opening for introducing a mixture of particulate matterand/or froth into the cell; a discharge orifice in a wall of the cell; afroth support in communication with the discharge orifice for receivingfroth from the cell and supporting the froth; and a wash sprayerdisposed upstream of the discharge orifice. If the apparatus is operatedin a semi-batch or continuous mode, then preferably the apparatus willinclude a liquid drain in a wall of the cell, for example in the lowerend of the cell.

Still another aspect of the disclosure is a froth cleaning apparatus foruse with a froth flotation cell that causes froth to collect at the topof the cell. The apparatus will generally include all of the elements ofthe apparatus described above, except that various elements (such as thewalls of the cell, the aerator, and other elements found in flotationcells) can be provided with the flotation cell. Thus, one embodiment ofsuch a cleaning apparatus includes a hood including a lower peripheraledge for interface with the top of a froth flotation cell; a dischargeorifice disposed in the hood; a froth support in communication with thedischarge orifice for receiving pulp and/or froth and supporting thefroth; and a wash sprayer disposed upstream of the discharge orifice.

Further aspects and advantages of the invention may become apparent tothose skilled in the art from a review of the following detaileddescription, taken in conjunction with the appended claims. While theinvention is susceptible of embodiments in various forms, describedhereinafter are specific embodiments of the invention with theunderstanding that the disclosure is illustrative, and is not intendedto limit the invention to the specific embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a froth flotation apparatusaccording to the disclosure.

FIG. 2 is a partial cross-sectional view of the froth cleaning sectionof the apparatus of FIG. 1.

FIGS. 3 and 4 show partial cross-sectional views of a froth cleaningsection of an apparatus according to the disclosure.

FIGS. 5 and 6 show partial cross-sectional views of a variation of afroth cleaning section of an apparatus according to the disclosure.

FIGS. 7 and 8 show partial cross-sectional views of a variation of afroth cleaning section of an apparatus according to the disclosure.

FIG. 9 is a partial cross-sectional view that shows the interface of anembodiment of a froth cleaning apparatus according to the disclosurewith a type of flotation cell having an overflow weir and launder aroundthe circumference of the cell, such as those known in the art.

FIG. 10 is a partial cross-sectional view of a froth cleaning apparatusaccording to the disclosure including a froth support having a movableupper section to control the depth of froth passing through the cleaner.

FIG. 11 shows a partial cutaway view of a froth support apparatus thatincludes a repositionable panel to change the height of a passage formedtherewith from the bottom to control the depth of froth passed throughthe passage.

FIG. 12 is a partial cross-sectional view of a froth flotation apparatusaccording to the disclosure that includes a froth cleaning sectionaround the circumference of the major body of a cell and a hood to allowa larger volume for froth cleaning.

FIGS. 13 and 14 show partial cross-sectional views of a variation of afroth cleaning section of an apparatus according to the disclosure.

FIG. 15 shows a cross-section of a froth flotation apparatus accordingto the disclosure that includes a generally zigzag-shaped froth cleaningsection.

FIG. 16 shows a froth cleaning apparatus according to the disclosureinterfaced with a laboratory-scale flotation column.

FIG. 17 shows a cross-sectional view of the cleaning section of theapparatus in FIG. 16, showing a grooved lower support surface.

FIGS. 18 and 19 are charts comparing ash and sulfur rejectioncapabilities, respectively, of a conventional flotation column withfroth washing and a conventional flotation column equipped with a frothcleaning apparatus according to the disclosure. The curves shown in thetwo figures plot the best possible cleaning performance according toAdvanced Flotation Release analysis, as described below.

FIGS. 20 and 21 are charts comparing sulfur and ash rejectioncapabilities, respectively, the same apparatus used to generate FIGS. 18and 19.

FIG. 22 shows the results of carrying capacity tests for an apparatusaccording to the disclosure.

FIG. 23 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of Illinois fine coal (8%ash) in AFR analysis and with a subaeration cell modified according tothe disclosure.

FIG. 24 plots combustible recovery versus total sulfur content of theproduct separated from a slurry (5% solids) of Illinois fine coal (3.17%sulfur) in AFR analysis and with a subaeration cell modified accordingto the disclosure.

FIG. 25 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of a reconstituted feed (24%ash) in AFR tests and with a subaeration cell modified according to thedisclosure.

FIG. 26 plots combustible recovery versus total sulfur content of theproduct separated from a slurry (5% solids) of a reconstituted feed(3.44% sulfur) in AFR tests and with a subaeration cell modifiedaccording to the disclosure.

FIG. 27 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of high-ash Illinois coalrefuse (40% ash) in AFR tests and with a subaeration cell modifiedaccording to the disclosure.

FIG. 28 plots combustible recovery versus total sulfur content of theproduct separated from a slurry (5% solids) of Illinois coal refuse(3.7% sulfur) in AFR tests and with a subaeration cell modifiedaccording to the disclosure.

FIG. 29 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids; 24% ash) of fine coal mixedwith washing plant rejects (tailings) in AFR tests, with a subaerationcell modified according to the disclosure, and with a 1.3 ft³subaeration cell modified according to the disclosure to process a pulp(3% solids) containing fine coal with 26% ash drawn from the rejectstream of an Illinois coal preparation plant.

DETAILED DESCRIPTION OF EMBODIMENTS

The invention generally relates to a method and apparatus for separationand concentration of particulate matter, and is particularlyadvantageous for beneficiation of coal fines.

One aspect of the disclosure is a method of separating a desiredconstituent from a mixture of particulate matter, including the steps ofaerating a pulp to generate a float fraction of froth supported on thesurface of a non-float fraction of pulp, separating a portion of frothfrom the float fraction, draining the separated froth, washing theseparated froth to dislodge undesired constituents, and recovering thewashed froth.

The mixture of particulate matter is combined with a liquid and afrothing agent to create a pulp. The frothing agent stabilizes bubblesthat are created by the introduction of air into the pulp. For practicalreasons, the major liquid component of the pulp typically will be water,though this need not be the case.

The mixture of particulate matter is not limited to any specific type ofcompounds. Typical applications can include mineral processing and wasteprocessing. Examples include separation of coal from mixtures includinggangue materials such as sulfur, pyrite, and other ash-formingmaterials; separation of cellulosic (e.g., paper) fibers from mixtureswith ink; separation of dolomite from phosphate-containing minerals;separation of hematite from quartz; separation of chalcopyrite and/orchalcocite from silicate gangue minerals; separation of zeolite fromquartz; separation of quartz from magnesite; separation of calcite fromapatite; separation of feldspar from nepheline syenite; and separationof silica-containing minerals from limestone.

The mixture of particulate matter is not limited to any specificparticle sizes; however, the method and apparatus disclosed herein offersignificant advantages over known methods of processing mixtures withvery fine particle sizes, such as less than about 0.65 mm.

A frother (frothing agent) can be added to promote the formation ofstable bubbles under aeration. A frother preferably includes both apolar end and a nonpolar end. The type and amount of frothing agentsuitable will vary with the mixture of particulate matter and theconstituent desired to be separated, similar to known art. Frothers aregenerally classified by their polar groups, with the most common beinghydroxyl, carboxyl, carbonyl, amino, and sulfo groups. Preferably, thefrother will contain at least five or six carbon atoms in astraight-chain, nonpolar group for sufficient and stable interactionwith the gaseous (e.g., air) phase. For branched-chain hydrocarbons, thenumber of carbon atoms in the nonpolar group preferably is about 16 orless. For example, methyl isobutyl carbinol (MIBC) is a suitablefrothing agent for use with coal. Often, a suitable type and amount offrothing agent cannot be accurately predicted for a particular type ofore, but can quickly be determined by empirical testing. The type offrothing agent is typically a secondary consideration, chosen after acollector (described below), to provide suitable frothing conditionswithout interfering with a collector or separation system (i.e.,combination of a collector and other optional reagents).

Optionally, the pulp can contain one or more reagents selected fromcollectors, activators, depressants, dispersants, and other modifiers(e.g., pH modifiers). Collectors are used to invoke selectivehydrophobicity, and are typically heteropolar organic substances. Thenonpolar end is almost always a long-chain or cyclic hydrocarbon groupthat is hydrophobic. Preferably, a suitable collector will selectivelyinduce hydrophobicity on the desired material to be recovered whileretaining hydrophilicity of the nondesirable material. Both anionic andcationic collectors can be used. Examples of anionic collectors includesodium oleate, xanthates, dithiophosphates, alkyl sulfuric salts. Themost common cationic collectors include amine groups, such as anilineand pyridine, and are optionally used with an acid to induce solubility.Because of the inherent heteropolarity of collectors, a collector canalso serve as a frother in some systems (e.g., sodium oleate andsulfo-soliophil fatty acids).

The natural hydrophobicity of coal (especially freshly ground coal) isan asset which minimizes the use of collectors. However, on extendedstorage, or if the coal or coal seam has been in contact with air for afew years (even days, in some circumstances), the coal can tend to losemuch of its hydrophobicity, and is often then referred to as “weatheredcoal.” The addition of a collector, such as hydrocarbon oils (e.g.,kerosene or fuel oil), can make coal more floatable. Typically-usedalcohols include MIBC, and amyl, hexyl, heptyl and octyl alcohols.Typical amounts of oils and alcohols per 1 ton of coal are about 50gr/ton to about 250 gr/ton and about 250 gr/ton to about 1000 gr/ton,respectively.

Activators can be added to chemically “resurface” the solid to increaseinteraction with collectors that are otherwise ineffective alone.Depressants form a polar chemical envelope around a solid particle thatenhances hydrophilicity or selectively prevents interaction withcollectors that might induce unwanted hydrophobicity. Dispersants act tobreak agglomerated particles apart so that single particles interactwith the collector and air bubbles. A pH modifier (e.g. an alkalinemodifier such as caustic soda, lime, and soda ash; and an acidicregulator such as hydrochloric and sulfuric acids) can be used, becausethe hydrophobicity of systems is often optimal within a particular pHrange, and some frothers require a certain pH range to form stablebubbles. A pH modifier can, in some cases, also serve as one or more ofa dispersant, a depressant, and an activator.

The pulp is aerated to generate a float fraction of froth supported onthe surface of a non-float fraction of pulp. The float fraction includesa plurality of bubbles, at least a portion of which are selectivelyattached to a desired constituent of the pulp. Aeration can be achievedby various techniques, including a sparger, impeller, agitator, and thelike, disposed within the pulp. As used herein, the term “aerate” andforms thereof are defined to relate to not only air but also any othergaseous substance. For example, nitrogen bubbles have been used toeffect the separation of copper sulfide minerals from molybdenum sulfideminerals. In a preferred embodiment, aeration is achieved via theintroduction of air through the shaft of an agitator, and exits into thepulp at the bottom of the agitator (i.e., subaeration). In anotherembodiment, aeration can be achieved via an aerator that includes aninjector jet disposed in a conduit and a source of air in communicationwith the conduit, such as disclosed in U.S. Pat. No. 4,938,865 (Jul. 3,1990), the disclosure of which is incorporated herein by reference. Byuse of such an aerator, air can be entrained into liquid pulp, forexample to create a downward-moving foam bed in a column that entersfrom the lower part of the column into a vessel, in which the frothseparates from liquid, forming a liquid(pulp)/froth interface.

By use of a method and apparatus according to the disclosure, the amountof air required by the system is surprisingly substantially reducedcompared to known methods. For example, a 100 ft³ (about 2.8 m³)subaeration cell with an open top would typically require about 300ft³/min (about 8.5 m³/min) of air supply (e.g., from a blower), whereasa similarly-sized cell according to the disclosure herein that includesa closed top would require only about 15 ft³/min (about 0.4 m³/min) ofair supply.

The location (typically, the level within a vessel) of the liquid/frothinterface is a variable that can be adjusted by the operator of aflotation cell. In a preferred method according to the disclosurewherein very fine froth is produced, the liquid/froth interface isformed near the nominal “top” of the primary enclosure of the vessel,more preferably at or above the top. Thus, in one embodiment, theliquid/froth interface is formed at a position slightly above the top ofthe primary enclosure of the vessel, such that the area of mixing withinthe apparatus is further separated, preferably isolated, from the areaof particle separation. Very fine froth can be produced by variousmethods, such as by using a high concentration of flotation reagent, orby using an emulsified flotation reagent, such as EKOFOL 440 kch reagentavailable from EKOF Flotation GmbH of Bochum, Germany. This aspect ofthe preferred methods is described below in connection with embodimentsof the apparatus disclosed herein.

A portion of the froth is substantially separated from the floatfraction of froth. Thus, in one embodiment of the method, froth risingfrom the liquid/froth interface continuously pushes already-formed froth(in layers above) up the slope of an upwardly-inclined surface of afroth washer, such that the already-formed froth is then supported, atleast in part, by the upwardly-inclined surface of the froth washer. Theproduct-laden bubbles escape from the surface of the slurry and move upthe surface of the support. Without such a separation, thealready-formed froth would continue to rise, and as more froth isgenerated below the weight above would tend to reach a critical valueand destabilize a portion of the froth layer. In another embodiment,froth from the float fraction can be separated in other ways, includingmechanical methods, such as by pickup and conveyance along a moving beltdisposed, for example, at, adjacent to, or above the liquid/frothinterface. As another example, froth can be separated via other pressuredifferentials (e.g. discharge by force from produced froth, pneumaticconveyance such as from higher air pressure in the cell, suction, andthe like). Other ways of achieving separation will be apparent to thoseof skill in the art in view of the disclosure, and the disclosure is notmeant to be limiting.

Furthermore, separation can facilitate improved washing of the froth. Inknown processes wherein froth supported on a layer of pulp is washed,the non-selectively attached particles travel downwardly (i.e., in thedirection of gravity) through layers of froth below, increasing theprobability that the particles washed from the top layer will attach toany one of the layers of bubbles below. While in theory the flow offroth in such processes over a simple weir according to the prior artwill naturally tend to include froth from the top layers of the floatfraction, this is not always the case, and froth containing reattachedparticles also overflows the weir. In addition, even particles that arenot attached to froth tend to be carried over the weir with overflowingfroth. In contrast, by practice of a method described herein wherein aportion of froth from the float fraction is separated, the opportunitiesfor washed particles to reattach to discharged froth are very muchreduced, and the yield of desired constituents of the pulp issurprisingly increased.

Moreover, separation of the froth from the float fraction, canoptionally allow for drainage of the froth at various phases of theprocess. As the froth is drained of liquid, nonselectively-attachedparticles and particles that might otherwise be carried over into theproduct are removed (e.g., entrapped and entrained particles). Thus, forexample, as a portion of froth is pushed into and up the froth washingchute described below, the froth is substantially drained of pulp liquidand particles (e.g., nonselectively-attached particles) beginning at thetime of separation by such liquid running down the surface of theupwardly-inclined froth support (under the froth) and back into thecell. Thus, a froth drainer can include an upwardly-inclined frothsupport, a froth support including a perforated region for moreefficient separation of wash liquid (e.g., porous or containing otherholes, such as a screen), an entirely perforated support, and the like.

Preferably, the froth is at least partially drained before washing,preferably substantially drained. In addition, if the froth issubsequently washed in such an apparatus, then the froth preferably willbe drained of wash water and particles both during and subsequent towashing.

A method of separating or concentrating a desired constituent from amixture of particles as described herein includes a step of washingseparated froth with a liquid to dislodge undesired particles from thefroth. The undesired particles typically will include non-selectivelyattached, entrained, and entrapped particles, and mixtures thereof. Thewash liquid preferably includes as a major component a liquid that isalso used in the pulp, for convenience and compatibility. Thus, when thepulp includes water, then preferably the wash liquid is water. The washliquid can also include one or more additional agents, such as afrother, collector, activator, depressant, dispersant, or othermodifier. Preferably, the washing step includes more than one washingoperation, and more preferably includes a plurality of washingoperations in series, such that each portion of separated froth iswashed more than once.

The better the separation of the wash liquid from the separated froth,the lower is the risk that dislodged particles in the wash liquid willreattach to bubbles in the separated froth as the wash liquid is drainedfrom the froth. Thus, the wash liquid that includes dislodged particlespreferably is substantially separated from contact with the separatedfroth, and more preferably is also separated from the washed froth. Thiscan be achieved for example, via an apparatus described below whereinthe separated froth travels up an upwardly-inclined surface, such thatthe wash water flows down the upwardly-inclined surface and has theopportunity to contact only the lowermost layer of bubbles in theseparated froth. Preferably, such an upwardly-inclined support can beprovided with a nonplanar (e.g., toothed or crenate, crenellated,corrugated, and the like) surface profile, such that the wash liquidcollects and travels at the lower extremities of the profile, furtherseparating the froth from the wash liquid. Other ways of achieving suchseparation will be apparent to those of skill in the art in view of thedisclosure, and the disclosure is not meant to be limiting.

As described above, the depth of a layer of froth can affect thestability of the bubbles at the bottom of the froth, and can also affectthe effectiveness of washing the froth. For example, the deeper a massof froth (more layers), the more likely it is thatnonselectively-attached particles washed from a top layer will reattachor become entrapped in a lower layer of froth. On the other hand, as thedepth of froth decreases, the throughput of the process tends todecrease. In addition, the ability to reject fine contaminant particlesappears to decrease with increasing depth, especially for fine particlessuch as clay and pyrite that have relatively slow settling velocities.Thus, the depth of the separated froth preferably is controlled toachieve a desired balance between factors including those describedabove. The depth of froth can be controlled, for example, by controlling(e.g., fixed or variable) the height of an opening through which frothis discharged from a flotation chamber or, in addition, by forcing froththrough a conduit having a controlled (fixed or variable) height. Inpreviously-known froth flotation apparatus, the froth depth must be ofsufficient height to permit froth to flow over a weir withoutsignificant carryover of pulp. In contrast, in a method and apparatus asdisclosed herein, the froth depth can be relatively smaller, because aportion of the float fraction is separated from the pulp and can bewashed and drained while separated. Accordingly, provision of a frothsupport allows the effective height of a froth column to be increasedwithout affecting the stability of the lower layers of the froth,because much of the mass of the froth is supported by the froth supportrather than the underlying bubbles.

The washed froth can be recovered by collecting the froth in a launderas it is discharged at the outlet of a froth washing apparatus or viaflow over a weir, with or without mechanical skimming, for example.

The hydrophilic particles remain in the pulp solution, which iseventually decanted off as waste (e.g., a tailing stream), in either abatch, semi-batch, or, preferably, a continuous process. Typically theconcentrated stream contains at least one desired, or more valuable,component and the tailing stream contains one or more less valuablecomponents (e.g., gangue). In some applications, however, the morevaluable component will be in the tailing stream and a less valuablecomponent will be desired to be removed in the concentrated stream fromthe froth.

Another aspect of the disclosure is an apparatus for separating adesired constituent from a mixture of particulate matter, the apparatusincluding at least one wall (e.g., a sphere), preferably walls, defininga flotation cell (e.g., a cylindrical vessel); an aerator for aerating amixture of particulate matter to produce froth; a feed opening forintroducing a mixture of particulate matter and/or froth into the cell;a discharge orifice in a wall of the cell; a froth support incommunication with the discharge orifice for receiving froth from thecell and supporting the froth; and one or more wash sprayers disposedupstream of the discharge orifice. If the apparatus is operated in asemi-batch or continuous mode, then preferably the apparatus willinclude a liquid drain in a wall of the cell, for example in the lowerend of the cell. At least a portion, preferably the majority, of thefroth support is disposed at or above the top of the flotation cell. Inoperation, at least a portion, preferably the majority, of the frothsupport is disposed at or above the froth/liquid interface.

Still another aspect of the disclosure is a froth cleaning apparatus foruse with a froth flotation cell that causes froth to collect and/or beconfined at the top of the cell. The apparatus will generally includeall of the elements and aspects of the apparatus described above, exceptthat various elements (such as the walls of the cell, the aerator, andother elements found in known flotation cells) can be provided with theflotation cell. Thus, one embodiment of such a cleaning apparatusincludes a hood including a lower peripheral edge for interface with thetop of a froth flotation cell; a discharge orifice disposed in the hood;a froth support in communication with the discharge orifice forreceiving froth (and, optionally, pulp) and supporting the froth; andone or more wash sprayers disposed upstream of the discharge orifice.

Either of the above-described apparatus will preferably include one ormore features selected from agitators, froth depth controllers, frothmotivators (mechanical or otherwise), mesh screens, and perforatedplates (for example, disposed in the path of travel of at least aportion of the generated froth).

In a froth flotation apparatus according to the disclosure, the wall orwalls defining the flotation cell preferably will define a substantiallycylindrical vessel (column) that, in contrast to known flotation cells,is at least substantially closed at the top. The cylinder can have acircular, elliptical, square, rectangular, or any other shape of crosssection. The length (height) of the column will typically be greaterthan the width.

The aerator serves to introduce a gas into a mixture of particulatematter to produce bubbles. The aerator can be selected, for example,from a diffuser, sparger, agitator, and impeller disposed in or indirect contact with the pulp. In a preferred embodiment, aeration isachieved via the introduction of air through the shaft of an agitator,and exits into the pulp at the bottom of the agitator (e.g., asubaeration cell). The aerator can also be located outside a body ofpulp, for example a diffuser or sparger that introduces pre-aeratedliquid (e.g., water) from outside the flotation compartment. In anotherembodiment, the aerator located outside a body of pulp can include aninjector jet disposed in a conduit and a source of air in communicationwith the conduit, such as disclosed in U.S. Pat. No. 4,938,865,described above.

A froth flotation apparatus includes a discharge orifice in a wall ofthe cell. Similarly, a froth cleaning apparatus includes a dischargeorifice disposed in the hood. The orifice provides a path for frothand/or pulp to exit the cell. When the top of the orifice extends abovethe froth support to form a passage of controlled height (e.g., to forma flap or hood and the like over a froth support (e.g., a discontinuouscross-sectional perimeter) or to form a conduit, channel, chute, orpipe, and the like (e.g., a continuous cross-sectional perimeter) with afroth support), it can also serve as a froth depth controller, forexample if froth is generated within the cell and exits through theorifice at an angle (i.e., not vertically).

In either the froth flotation apparatus or the froth cleaning apparatus,a froth support is in communication with the discharge orifice forreceiving and supporting froth. The froth support can take any formsuitable for conveying and, preferably, draining froth of the desireddepth. The froth support can be a single member or an apparatusincluding a plurality of individual members. Suitable examples include aclosed cylindrical conduit and a closed conduit having a rectangularcross-section (e.g., a chute). Preferably, the froth support includes abottom interior surface (e.g., a major surface supporting the froth)that has an average width greater than the average height of thesupport. Thus, the width of the conduit will be greater than the heightof the conduit, for example.

At least a portion, preferably the majority, of the froth support isdisposed at or above the top of the flotation cell. In operation, atleast a portion, preferably the majority, of the froth support isdisposed at or above the froth/liquid interface. Accordingly, suchembodiments will facilitate drainage of the froth, as opposed to frothsimply flowing over a weir as in the prior art.

In one embodiment, the froth support includes a froth support surfacethat has a non-planar profile. As described above, such a feature canserve to promote drainage and separation of froth from drained pulpand/or wash water. Examples include crenate, crenellated, and corrugatedprofiles. The profile can also be substantially smooth and concave (todirect drained liquid to the center) or convex (to direct drained liquidto the edges).

In another embodiment, the froth support includes a froth supportsurface that is perforated (e.g., a perforated or porous plate or ascreen) to promote drainage of froth and removal of liquid while stillsupporting the froth, or at least a majority of the bubbles of thefroth. Such a perforated froth support can include a channel, conduit,or the like for collecting drained liquid for return to the flotationcell or recycle. For example, the collected drained liquid can berecycled to make up additional pulp, or can be filtered to provide arecycle wash water stream. Alternatively, the collected drained liquidcan be discharged from the process.

When the froth support has a continuous cross-sectional perimeter (e.g.,to form a substantially closed channel, chute, conduit, or the like),such that gas (e.g., air) introduced into the cell acts as a drivingforce, especially when it is the sole driving force, for froth to movethrough the support then preferably the support includes a vent forrelease of air. The vent preferably is located at the top of thesupport. The distance of the vent from the discharge opening of thecell, and the size of the vent, will tend to affect pulp and froth flowdynamics. For example, for a pulp in which a relatively high frequencyof collision between bubbles and pulp particles is desired, then a highflow rate of air will be desired in the cell, and a relatively largevent and/or a vent located closer to the main body of the cell will bepreferred. For an application wherein a relatively long froth support isdesired to facilitate cleaning of the froth, then a vent, when used,will preferably be located relatively far from the discharge from thecell, such that the froth has sufficient motivation force behind it topropel it through the support. Accordingly, in a preferred embodiment,one or more of the vent size and the vent location is adjustable. Thevent may be a simple hole or can advantageously include a conduit (e.g.,a stack or chimney) to prevent substantial loss of froth through thevent. Such a stack can include a damper to permit control over thevolume of gas flow.

When the froth support includes a conduit, the conduit can include oneor more features to control the depth of froth. For example, the conduitcan have a tapered cross section, wherein the end of the conduit inproximity to the first discharge outlet in the wall of the flotationcell or in the hood can have a relatively larger cross-section than theopposite end of the conduit. Such a feature can serve to increase thedepth of the froth in the case where a portion of the bubbles in thefroth are destabilized during washing. A tapered conduit canalternatively have the reverse configuration, wherein the end of theconduit in proximity to the first discharge outlet in the wall of theflotation cell or in the hood has a relatively smaller cross-sectionthan the opposite end of the conduit. In such an embodiment, the depthof froth will decrease as the froth progresses through the conduit, suchthat the froth depth is at a minimum during its final washing, wherebythe opportunity for reattachment or entrapment of dislodged particles isminimized and thereby a concentrate of high purity is obtained.

The froth support can also include one or more movable parts to controlthe depth of froth. For example, in an embodiment described below, anupper panel of a froth chute can be selectively positioned relative tothe lower panel of the froth chute, to provide a variable height in thechute. Similarly, the bottom panel of such a chute can also be adaptedfor variability.

The froth support preferably is disposed at an upwardly-inclined angle,such that froth is conveyed up the support. For example, if theflotation cell includes a base having a major plane, the froth supportpreferably is disposed at an upwardly-inclined angle with respect to themajor plane of the base. As for the froth cleaner apparatus adapted foruse with a flotation cell, when the lower peripheral edge of the hoodsubstantially lies in a plane, the froth support can be disposed at anupwardly-inclined angle with respect to that plane. Put another way, thefroth support preferably is disposed at an angle greater than 90 degreeswith respect to the gravity vector when the apparatus is in operation.In similar fashion, when either of the apparatus is in operation, thefroth support preferably is disposed at an upwardly-inclined angle withrespect to the liquid/froth interface. For example, when concentratingcoal fines in an apparatus wherein the froth height is controlled toabout 1 inch (about 2.54 cm), then preferably the support angle is about25 degrees, e.g. 22.5 degrees.

As the froth height is increased, then the angle preferably is alsoincreased. As a result of inclination, the distance that the froth musttraverse before being removed from the apparatus is generally greaterfor a given total froth depth than for a traditional vertical column offroth of the same depth, but the static pressure on the lower layers ofthe froth is less than a vertical column of froth of the same height.When operating conditions in the cell present the possibility that toomuch contaminant material will be carried over with the concentrate, ahigher angle of inclination can be used. This could be the case, forexample, when using a more coarse material and/or when the slurry levelis raised and the column of froth within the cell (as opposed to thecleaner section) is kept to a minimum. Under some conditions, the slurrylevel can actually rise into the lower part of the upwardly-inclinedfroth cleaner with the froth bearing the product particles emerging fromthe slurry within the froth cleaning device. Stable froth conditions inthe cell may also allow the operator to lower the slurry level, maintaina thicker froth column in the cell, and lower the angle of upwardinclination of the washer. Adjustment of the angle of upward inclinationof the washer gives the operator another option to control theperformance of the cell for given feed materials, rates of throughput,and other variables.

One class of froth support embodiments designed to cope with potentialintermittent surges of pulp and concomitant or resulting changes in theposition or level of the liquid froth interface includes a non-linearpathway for fluid travel. Thus, for example, when a flotation cellincludes a base having a major plane, then the froth support includesfirst and second sections disposed at upwardly-inclined angles (same ordifferent) with respect to the major plane of the base and the first andsecond sections are disposed in opposing directions with respect to aplane perpendicular to the major plane of the base. As for the frothcleaner apparatus adapted for use with a flotation cell, when the lowerperipheral edge of the hood substantially lies in a plane, the frothsupport includes first and second sections disposed at upwardly-inclinedangles (same or different) with respect to the lower peripheral edge andthe first and second sections are disposed in opposing directions withrespect to a plane perpendicular to the lower peripheral edge.

Preferably, the support has a convoluted path for fluid travel (e.g.,tortuous, serpentine, winding, or boustrophedonic). If the pulp levelintermittently rises (e.g., in surging fashion), then one or moredeviations in fluid path flow can aid in preventing the surging pulpfrom contaminating cleaned froth, for example by spilling over a weirinto a launder. Such a support preferably includes one or more openingsand/or vents for discharge of air. A support having a convoluted pathfor fluid travel thus can provide a path for easy escape of excess airor gas introduced into a flotation cell without causing excessivespillage from froth from the washer before it can be adequately cleaned.

In an alternative arrangement, the support can be constructed in such away that its overall length may be increased or reduced (e.g., via atelescopic mechanism) in which one part of the support slides insideanother, or by the addition or subtraction of segments of support (e.g.,with the same cross-sectional area at segment interfaces), and of aconvenient incremental length.

In either the froth flotation apparatus or the froth cleaning apparatus,one or more wash sprayers (e.g., nozzles) are disposed upstream of thedischarge orifice. A wash sprayer can be disposed above the surface ofthe froth support adapted to support froth, preferably in the upper walland/or side walls of a froth chute. The wash sprayer serves to dispensewash liquid onto and/or into the froth as it is conveyed along the frothsupport, for example in a cone or plane configuration. Preferably, thewash sprayer will provide a screen of wash liquid through which thefroth passes. Thus, preferably, a sprayer is disposed tangential to theflow of froth, and above the froth support. When more than one washsprayer is used, preferably the sprayers are disposed in series, suchthat each portion of froth is washed more than once. When each sprayerdoes not provide wash liquid to the entire height and/or width of froth,the wash sprayers can be disposed in rows or in staggered relationshipsuch that the entire height and/or width of froth undergoes at least oneapplication of wash liquid. In an embodiment such as that described inthe Examples below, the turbulence of wash spraying can be increased toa point that would destroy the froth in a typical vertical flotationcolumn; however, due to the very wet and flowing conditions present inthe cleaner section, the froth simply becomes more fluid and clean.

The froth support preferably includes a weir over which the washed frothcan flow into a launder for recovery.

Either the froth flotation apparatus or the froth cleaning apparatus canalso include one or more screens (e.g., a mesh screen) or perforatedplates through which a portion of generated froth can travel. Forexample, the apparatus can include a screen disposed in the path oftravel of separated froth in proximity to and/or upstream of thedischarge orifice. A screen can serve to separate bubbles to facilitaterelease of undesired particles entrapped between bubbles, and can helpto deter the flow of liquid pulp (e.g., surging pulp), for example toprevent liquid pulp from flowing out through the froth washing deviceand contaminating washed product. In one embodiment, the froth supportincludes two plastic plates perforated with 0.25-inch (6.4 mm) diameterholes, the plates disposed a short distance (e.g., about an inch) apart.The holes in the plates had offset centers, which forced the froththrough a tortuous path while partially blocking surging pulp fromtravelling high up into the washing section of the support.

Either of the apparatus can also include one or more froth motivators,including a mechanical froth motivator such as those described below inconnection with the figures, to assist in conveyance of froth upstreamof the discharge orifice.

A froth flotation apparatus equipped with a froth cleaning system asdisclosed herein can be used alone and can also be used in series, forexample whereby the cleaned product and/or the tail stream can be fed toa subsequent flotation apparatus.

Specific embodiments are described below in connection with the figures.

FIG. 1 depicts a froth flotation apparatus that includes a wall 12defining a flotation cell 10 containing a pulp 14 that enters through afeed opening 18 via a feed conduit 20. An agitator 22 driven by a motor24 is disposed within the interior of the flotation cell 10. Air 28 isfed through the shaft 30 of the agitator 22 for release into the pulp 14at the bottom of the agitator 22 to achieve subaeration. Pulp containinga relatively higher concentration of undesired particles is dischargedfrom the cell through orifice 32 and conduit 34 at the bottom of thecell 10.

The apparatus also includes a froth cleaner section 38 including a frothsupport 40 disposed at an angle α with respect to the major plane of thebase 42 of the flotation cell 10. Pulp and/or froth enters the cleanersection 38 via orifice 44. The support 40 is in the form of arectangular chute, with an overflow weir 48 leading to launder 50. Washsprayers 52 are disposed in a top panel 46 of the chute in series tospray wash liquid on froth 54. Air 28 introduced through the agitator 22exits through opening 56. The apparatus in FIG. 1 is shown in a mannerof operation wherein the liquid/froth interface 58 is at the top of thecell 10 and, thus, within the cleaner section 38. Operation in thismanner is preferred to achieve high throughput and vigorous agitation inthe interior of the cell 10.

FIG. 2 is a cross-sectional view of the froth cleaning section 38 at thelocation indicated on FIG. 1, wherein like reference numbers indicatelike elements. FIG. 2 shows a sprayer 52 disposed above a crenate(toothed) bottom panel 60 of the froth support 40. In this embodiment,wash liquid will preferentially flow in areas 60 a of the froth support,whereas the froth will tend to be supported above.

Another advantage of this embodiment is that it includes no more movingparts than a traditional flotation column which washes the froth at thetop of the column and, particularly with respect to a subaeration cell,no additional moving parts in direct contact with the pulp, which can berather dense (e.g., up to 60% or more solids) and abrasive. As aconsequence, operating costs, and in particular the cost of maintenanceand repairs, are no greater than for a flotation column or subaerationcell without a froth cleaning device disclosed herein, whereas betterseparation is obtained.

FIGS. 3 and 4 show a partial view and a cross-sectional view,respectively, of a froth cleaning section 70 of a froth flotationapparatus according to the disclosure. This apparatus includes aplurality of mechanical froth motivators in the form of froth pushers 72attached to a belt 74 supported by spindles 78, one or more of which aredriven to rotate the belt. As a consequence of including frothmotivators, this apparatus can be disposed at a lower upwardly-inclinedangle. In the apparatus shown, wash sprayers 82 are disposed in sidewalls 84 and 86 (FIG. 4) of the froth support 90. The froth support 90is shown with a crenate bottom panel 92, and the pushers 72 are disposedsuch that an edge of each pusher 72 comes in proximity to or in contactwith the panel 92 at upper zones 92 b to move the majority of froth 94through the channel formed by the belt 74 and the support 90 in thelower region of the support 90. As in the embodiment shown in FIG. 3,wash liquid will preferentially flow in areas 92 a of the froth support.The support 90 includes an air exit opening 88 and an overflow weir 96leading to launder 98. The embodiment as shown inludes a top panel 76,though this need not be the case as long as froth is prevented fromescaping the apparatus via other means, such as by provision of asealing flap (not shown) which can be disposed at region 80, forexample, to interface with the belt 74 but still allow passage of themotivators 72.

FIGS. 5 and 6 show another variation of an apparatus including a frothcleaning section 99, FIG. 5 being a partial view and FIG. 6 being acorresponding cross-sectional view. This apparatus also includes aplurality of mechanical froth motivators in the form of froth pushers100 mounted on a belt 102 on spindles 104 and including a series of washsprayers 108. In this embodiment, the belt 102 and pushers 100 move inthe clockwise direction, as shown, such that the froth is moved throughthe channel formed by the belt 102 and the support 110 in the upperregion of the support 110, and the sprayers 108 are disposed in a top112 of the support 110 to wash the froth that passes by.

The pushers 100 in this embodiment preferably intersect with the support110 at the bottom panel 114 in a substantially sealing fashion, suchthat only insubstantial amounts of pulp and/or froth are allowed toenter the bottom channel formed by the belt 102, the bottom wall 114,and side walls 118 and 120 of the support 110 in a direction counter tothe belt movement. For example, pushers 100 can be constructed of aflexible material and in a length slightly greater than the bottomchannel, such that the pushers 100 bend when traveling through thebottom channel. In the embodiment shown, the belt 102 is shown in FIG. 4to have grooves 122 (shown with a triangular cross-section) throughwhich wash water can flow back into a flotation cell (not shown). Theweir 124 in such an embodiment is positioned in such a manner that atleast a majority of the washed froth 126 flows into the launder 128 andair exits through opening 130.

FIGS. 7 and 8 show another variation of an apparatus including a frothcleaning section 132, FIG. 7 being a partial view and FIG. 8 being acorresponding partial cross-sectional view. This apparatus includes aseries of wash sprayers 134 (disposed in a top panel 136) and aperforated plate 138 in the support 140. Wash liquid passes through veryfine holes 142 in the plate 138 to facilitate drainage of liquid fromthe froth 144 without substantial loss of froth 144 from the support140. A bottom channel 146 formed by a bottom panel 150 and side panels152 and 154 of the support 140 collects the wash water for return to theflotation cell (not shown), directly or indirectly, or for otherdisposal. The apparatus is provided with an air exit opening 156, andoverflow weir 158 that leads to a launder 160. The top panel 136 canextend to the vicinity of the overflow weir 158 or, in some embodiments,can extend only so far as to provide support for the wash sprayers 134.

FIG. 9 shows the interface of an embodiment of a froth cleaningapparatus 162 according to the disclosure with a type of flotation cell164 having an overflow weir 166 and launder 167 around the circumferenceof the cell 164. The apparatus 162 is shown before installation (inphantom lines) and after installation. The apparatus 162 generallyincludes a hood 168 including a lower peripheral edge 170 for interfacewith the top of the cell 164 and a top 186. The apparatus 162 alsoincludes a discharge orifice 172 in the hood 168 that leads to acleaning section 174 having wash sprayers 176, an air exit opening 178,an overflow weir 180, and a launder 182. The cleaning section 174 cantake any configuration, such as those described in FIGS. 1-8. Uponinstallation of the apparatus 162, the weir 166 and launder 167 aresealed to prevent leakage of pulp 184, and instead the washed froth 188passes to launder 182.

To provide clearance for the hood, apparatus such as a pulley 190 thatdrives an agitator 192 might need to be relocated higher on the shaft194 of the agitator 192, as shown in the figure wherein the formerposition of the pulley 190 is shown in phantom lines 198. Though notshown, the apparatus could be configured to make use of the existinglaunder 167 by conveyance of the washed froth 188 thereto.

FIG. 10 is a partial view of a froth cleaner section 200 including afroth support 202 having a movable upper section 204 to control thedepth of froth 208 passing through the cleaner 200. In such anembodiment, wash sprayers 210 can be connected to associated flexiblewash liquid supply lines 212 (e.g., hoses) to permit repositioning ofthe upper section 204.

FIG. 11 shows a partial cutaway view of a froth support apparatus 220that includes a repositionable panel 222 to change the height of apassage formed therewith from the bottom to control the depth of frothpassed through the passage. As shown, the panel 222 can be secured inone of a selection of grooves 224 provided in side walls 228 (one shown)of the support 220. The panel 222 is also shown with a lip 230 to directpulp and/or froth above the panel 222.

FIG. 12 shows a froth flotation apparatus according to the disclosurethat includes a froth cleaning section 240 around the circumference ofthe major body of a cell 242 and a hood 244 to allow a larger volume forfroth cleaning. Such a cleaning section 240 and hood 244 can also beindependently adapted for interface with an independent flotation cell,analogous to the apparatus described and shown in FIG. 9.

FIGS. 13 to 15 show more apparatus adapted to cope with potentialintermittent surges of pulp and concomitant or resulting changes in theposition or level of the liquid froth interface. FIG. 13 shows a partialcutaway view of a froth support apparatus 246 that includes a mechanicalfroth motivator 250 in the form of a spindle 252 with pushers 254. Thepushers 254 in this embodiment preferably intersect with the support 246at the bottom panel 256 in a substantially sealing fashion, such thatonly insubstantial amounts of pulp and/or froth are permitted to passthe motivator 250 through the bottom passage bounded by the spindle 252,the bottom panel 256, and side walls 260 and 262 of the support 246 in adirection counter to the rotation of the froth motivator 250 (see alsoFIG. 14). For example, pushers 254 can be constructed of a flexiblematerial and in a length slightly greater than the distance to thebottom panel 256 from the spindle 252, such that a pusher 254 bends uponintersection with the bottom panel 256, for example when perpendicularto the bottom panel 256. In one embodiment, the motivator 250 isdesigned or disposed to leave an open pathway between the top panel 264of the support apparatus 246. In another embodiment, the motivator 250is designed or disposed such that a pusher 254 intersects with the toppanel 264 of the support apparatus 246. One or more wash sprayers 266preferably are disposed at a location upstream of the froth motivator250.

The a froth flotation apparatus (not shown) can be operated with such asupport apparatus 246 in such a manner that the pulp level 270 is belowthe froth motivator 250, as shown, or the pulp level can be raised suchthat at least a portion of the motivator 250 intersects the pulp level270. In the alternative, the motivator 250 can be disposed at a locationin the support apparatus 246 at a location closer to the flotation cell,such that it at least partially intersects the pulp level 270, whereinthe pulp level 270 is similar to that shown in FIG. 13.

If the pulp level 270 intermittently rises in surging fashion, such amotivator 250 can aid in preventing the pulp 272 from contaminating thecleaned froth 274 or from spilling over the weir 276 into the launder278. Air exits through opening 280.

FIG. 15 shows a cross-section of a froth flotation apparatus 282according to the disclosure that includes a generally zigzag-shapedfroth cleaning section 284. The cleaning section 284 includes a firstsupport section 286 disposed at a first upwardly-inclined angle β withrespect to the major plane of a base 290 and a second support section292 disposed at a second upwardly-inclined angle γ with respect to themajor plane of the base 290. The first and second sections 286 and 292are disposed in opposing directions with respect to a planeperpendicular to the major plane of the base (e.g., a plane parallel tothe side wall 294). A wash sprayer 296 is disposed in the second supportsection 292 to spray wash liquid on the froth therein.

The apparatus 284 includes a third support section 300 disposed at adownwardly-inclined angle δ with respect to the base 290 of theflotation apparatus 282 for flow of the cleaned froth into a launder302. A vent 304 for discharge of air 306 is in fluid communication withthe third section 300, and can be located at any point along the thirdsupport section 300. The launder 302 can have an open top (e.g., at thelower end of the third support section 300, as illustrated) fordischarge of air in addition to, or instead of a vent 304.

If the pulp level 310 intermittently rises in surging fashion into thefirst support section 286, the non-linear design of the first and secondsupport sections 286 and 292 can break the momentum of the pulp 312 andaid in preventing the pulp 312 from contaminating the cleaned froth 314or from spilling over into the launder 302.

EXAMPLES

The following examples are provided to illustrate the invention but arenot intended to limit the scope of the invention.

Example 1

A froth cleaning apparatus according to FIG. 16, was constructed forinterface with a 2-inch MICROCEL flotation column (developed by Roe-HoanYoon of Virginia Polytechnic Institute and State University). The frothcleaning apparatus had a connecting conduit 320 and a washing conduit322. The washing conduit 322 was 410 mm in length and 25 mm square incross-section, with two internal grooves 324 on the lower face 328, asshown in FIG. 17.

A series of 3 wash sprayers (two shown, elements 330 and 332) weredisposed in series along the path of froth flow, and any combination ofthe sprayers could be used for dispensing wash fluid. In the experimentsreported in Examples 2 to 4 below, only two wash sprayers were used. Thefirst wash sprayer 330 was disposed 165 mm from, the outlet end of thewash conduit 322 and the second wash sprayer 332 was disposed 100 mmfrom the outlet end of the wash conduit 322.

The froth cleaning apparatus was attached to the main body of theMICROCEL column using a series of connectors and elbows (replacing thetop launder section of the conventional MICROCEL column) to facilitateeasy manipulation of the washer inclination at different angles. Theresults reported in Examples 2-4 were obtained by operating theapparatus at an angle of 22.5 degrees.

Example 2

FIGS. 18 and 19 are charts comparing sulfur and ash rejectioncapabilities, respectively, of the MICROCEL flotation column with aconventional froth washer and the MICROCEL flotation column equippedwith a froth cleaning apparatus according to Example 1 (two washsprayers in operation). The results were obtained from treatment of −48mesh particle size fraction Pittsburgh No. 8 coal refuse. The curves inFIGS. 18 and 19 represent predictions based on advanced flotationrelease tests, described below.

The results show that the column equipped with the apparatus of Example1 generated a coal product with an ash content as low as 8.0% at acombustible recovery level of 80%, compared with an ash content of 10%at the same combustible recovery level achieved by the conventionalflotation column and froth-washing system (see FIG. 18).

The results also show that the column equipped with the apparatus ofExample 1 provided superior separation performance in terms of sulfurrejection. At a combustible recovery of nearly 80%, the column equippedwith the apparatus of Example 1 reduced the total sulfur content tolevels ranging from 5.1% to 3.3%, whereas 4.1% was the lowest sulfurconcentration that could be achieved by the flotation column with aconventional froth washing system at the same recovery value (see FIG.19).

Example 3

FIGS. 20 and 21 are charts comparing ash and sulfur rejectioncapabilities, respectively, of the same apparatus used in Example 2, andfrom treatment of the −325 mesh particle size fraction of the samesample as used for Example 2.

These side-by-side tests (see FIG. 20) again show a superior sulfurrejection performance for the column equipped by the apparatus ofExample 1 (1.75% total sulfur content in the product) as compared to theconventional flotation column (2.0% total sulfur content in theproduct). The performance of the apparatus of Example 1 also appeared tobe superior for achieving a low ash product (better combustiblerecovery), but for higher ash products the conventional column providedbetter combustible recovery (see FIG. 21).

Example 4

Carrying capacity tests were carried out to determine the effect of pulpfeed rate (g/(min·m²)) on the rate of combustibel recovery (g/(min·cm²))feed) of a flotation column modified according to the disclosure. FIG.22 shows the ability of an apparatus modified according to thedisclosure to produce a larger amount of product per minute, on aconstant cross-sectional area of column basis, believed to be the resultof reduced bubble coalescence. As shown in FIG. 22, for the −325 meshparticle size fraction of coal refuse from the Pittsburgh No. 8 CoalSeam, the carrying capacity of the flotation column modified accordingto the disclosure was found to be 28% greater than the conventionalflotation column.

Example 5

Various tests were carried out with a rectangular cuboid pilotplant-sized (about 37 liters in volume, outer dimensions about 28 cm by43 cm by 51 cm tall) subaeration cell equipped with an apparatus similarto that shown in FIG. 9, referred to herein as the “Big Cell.” Asdescribed above, subaeration cells are a proven technology with a higherthroughput than other commercial flotation systems, and are moreversatile and flexible than any other commercial flotation system.Adaptation of the subaeration cell by installing an apparatus accordingto the disclosure eliminates many of the shortcomings of the subaerationcell, such as incomplete separation of the product from the pulp.

Advanced Flotation Release (AFR) analysis should represent the bestresults that can be obtained with any commercial flotation device. TheAFR analysis is a process developed by Southern Illinois University toestablish the limits of achievement of flotation devices with regards tocombustible recovery and reduction of ash-forming mineral matter andsulfur. A graph produced according to AFR analysis is established byfloating the coal in a typical subaeration cell under conditions thatwill guarantee the fullest recovery of all the combustible as a product.This product is then floated in a packed column under optimum conditionsand the material floated within a certain time interval is collected andanalyzed.

These tests conducted with the Big Cell, on feeds containing variousamounts of ash and sulfur, demonstrated ash and sulfur rejection ratesby the Big Cell better than the level predicted by AFR analysis.

Thus, FIG. 23 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of Illinois fine coal (8%ash) by AFR analysis and with the Big Cell. The figure shows that theBig Cell achieved about 80% combustible recovery with as low as 4% ashcontent in the product, which is better than approximately 70%combustible recovery predicted by AFR analysis.

FIG. 24 plots combustible recovery versus total sulfur content of theproduct separated from a slurry (5% solids) of Illinois fine coal (3.17%sulfur) by AFR analysis and with the Big Cell. As shown in the figure,the Big Cell achieved much lower sulfur content in the product than thatpredicted by AFR analysis for the same levels of combustible recovery.

FIG. 25 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of a reconstituted feed (24%ash) by AFR tests and with the Big Cell. In these tests, for this feed,the Big Cell approached the theoretical maximum ash rejection ofconventional devices predicted by the AFR analysis for a range ofcombustible recovery levels.

FIG. 26 plots combustible recovery versus total sulfur content of theproduct separated from a slurry (5% solids) of a reconstituted feed(3.44% sulfur) by AFR tests and with the Big Cell. As shown in thefigure, the Big Cell achieved much lower sulfur content in the productthan that predicted by AFR analysis for the same levels of combustiblerecovery.

FIG. 27 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of high-ash Illinois coalrefuse (40% ash) by AFR tests and with the Big Cell. As shown in thefigure, the Big Cell achieved higher-than-predicted combustible recoverylevels for higher ash content levels.

FIG. 28 plots combustible recovery versus total sulfur content of theproduct separated from a slurry (5% solids) of Illinois coal refuse(3.7% sulfur) by AFR tests and with the Big Cell. As shown in thefigure, the Big Cell achieved a higher combustible recovery level in theproduct than that predicted by AFR analysis for the same levels ofsulfur content in the product.

FIG. 29 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids; 24% ash) of fine coal mixedwith washing plant rejects (tailings) by AFR tests and with the BigCell. Plotted for comparison are data obtained from in-plant tests withthe Big Cell fed a slurry with 3% solids and 26% ash content extractedfrom the tailings outfall from a coal washing plant in Illinois.

Various embodiments of the inventions can provide differing advantages,based on the objectives desired to be achieved with a flotationoperation. For example, an embodiment of the invention adapted toretrofit a single subaeration cell can produce a cleaner coal productthan that achieved with a flotation column, but at the large throughputrate of a common subaeration cell. Thus, a single device can take theplace of a system of subaeration cell batteries in which the froth ortails from one cell are re-cleaned in a subsequent cell to generate aproduct of desired purity. An apparatus according to the disclosure canbe operated to produce a well-drained, dry froth or a relatively wet,water-laden froth, depending on the needs of the system and the capacityof an associated filtration system.

The foregoing description is given for clearness of understanding only,and no unnecessary limitations should be understood therefrom, asmodifications within the scope of the invention may be apparent to thosehaving ordinary skill in the art. For example, variables such asaeration gas velocity, bubble size, pulp temperature, impeller speed,collector dosage, frother dosage, feed percent solids, volumetric feedrate, washer inclination, number of wash sprayers, froth height, biasrate, aeration gas volumetric rate, wash fluid volumetric rate, washfluid velocity, wash fluid frother addition, and other properties knownto those skilled in the art and based on the disclosure herein, can becontrolled within desired ranges to affect the quality and speed ofseparation.

What is claimed is:
 1. A froth flotation apparatus for separating adesired constituent from a mixture of particulate matter, comprising: awall defining a flotation cell; an aerator for aerating a mixture ofparticulate matter to produce froth; a feed opening for introducing amixture of particulate matter and/or froth into said cell; a dischargeorifice in a wall of the cell; a froth support in communication withsaid discharge orifice for receiving and supporting said froth; a washsprayer disposed upstream of said discharge orifice; and a screendisposed in proximity to and/or upstream of said discharge orifice inthe path of travel of at least a portion of the generated froth.
 2. Theapparatus of claim 1, comprising a froth depth controller.
 3. Theapparatus of claim 2, wherein said froth depth controller is selectedfrom the group consist of said discharge orifice, said froth support,and combinations thereof.
 4. The apparatus of claim 2, wherein saidfroth depth controller is a variable froth depth controller.
 5. Theapparatus of claim 1, wherein said froth support comprises a bottominterior surface of a conduit having an average width greater than anaverage height.
 6. The apparatus of claim 1, further comprising a basehaving a major plane and wherein said froth support is disposed at anupwardly-inclined angle with respect to said major plane of said base.7. The apparatus of claim 1, wherein at least a portion of said frothsupport is disposed above said flotation cell.
 8. The apparatus of claim1, wherein said froth support has a convoluted path for fluid travel. 9.The apparatus of claim 8, wherein said froth support comprises a firstsupport section disposed at a first upwardly-inclined angle with respectto said major plane of said base; and a second support section disposedat a second upwardly-inclined angle, same or different, with respect tosaid major plane of said base, wherein said first and second sectionsare disposed in opposing directions with respect to a planeperpendicular to said major plane of said base.
 10. The apparatus ofclaim 1, wherein said froth support is said screen which a perforatedregion of said froth support.
 11. The apparatus of claim 1, comprising aplurality of wash sprayers.
 12. The apparatus of claim 1, furthercomprising a froth motivator.
 13. A froth flotation apparatus forseparating a desired constituent from a mixture of particular matter,comprising: a wall defining a flotation cell; an aerator for aerating amixture of particular matter to produce froth; a feed opening forintroducing a mixture of particulate matter and/or froth into said cell;a discharge orifice in a wall of the cell; a froth support incommunication with said discharge orifice for receiving and supportingsaid froth; and a wash sprayer disposed upstream of said dischargeorifice, wherein said froth support has a convoluted path for fluidtravel.
 14. The apparatus of claim 13, comprising a froth depthcontroller.
 15. The apparatus of claim 13, wherein said froth supportcomprises a bottom interior surface of a conduit having an average widthgreater than an average height.
 16. The apparatus of claim 13, furthercomprising a base having a major plane and wherein said froth support isdisposed at an upwardly-inclined angle with respect to said major planeof said base.
 17. The apparatus of claim 13, wherein at least a portionof said froth support is disposed above said flotation cell.
 18. Theapparatus of claim 13, wherein said froth support comprises a firstsection disposed at a first upwardly-inclined angle with respect to saidmajor plane of said base; and a second support section disposed at asecond upwardly-inclined angle, same or different, with respect to saidmajor plane of said base, wherein said first and second sections aredisposed in opposing directions with respect to a plane perpendicular tosaid major plane of said base.
 19. The apparatus of claim 13, furthercomprising a froth motivator.
 20. A froth flotation apparatus forseparating a desired constituent from a mixture particulate matter,comprising: a wall defining a flotation cell; an aerator for aerating amixture of particulate matter to produce froth; a feed opening forintroducing a mixture of particulate matter and/or froth into said cell;a discharge orifice in a wall of the cell; a froth support incommunication with said discharge orifice for receiving and supportingsaid froth; and a wash sprayer disposed upstream of said dischargeorifice, wherein said froth support comprises a froth drainer tosubstantially separate wash fluid containing dislodged particles fromcontact with one or more of separated froth and washed froth, saidfroth, said froth drainer selected from the group consisting of aperforated region of said froth support, a nonplanar surface profile ofsaid froth support and combination thereof.
 21. The apparatus of claim20, comprising a froth depth controller.
 22. The apparatus of claim 20,when said froth support comprises a bottom interior surface of a conduithaving an average width greater than an average height.
 23. Theapparatus of claim 20, further comprising a base having a major planeand wherein said froth support is disposed at an upwardly-inclined anglewith respect to said major plane of said base.
 24. The apparatus ofclaim 20, wherein at least a portion of said froth support is disposedabove said flotation cell.
 25. The apparatus of claim 20, wherein saidfroth support has a convoluted path for fluid travel.
 26. The apparatusof claim 25, wherein said froth support comprises a first supportsection disposed at a first upwardly-inclined angle with respect to saidmajor plane of said base; and a second support section disposed at asecond upwardly-inclined angle, same or different, with respect to saidmajor plane of said base, wherein said first and second sections aredisposed in opposing directions with respect to a plane perpendicular tosaid major plane of said base.
 27. The apparatus of claim 20, furthercomprising a froth motivator.
 28. The apparatus of claim 20, furthercomprising a screen disposed in proximity to and/or upstream of saiddischarge orifice in the path of travel of at least a portion of thegenerated froth.
 29. A froth flotation apparatus for separating adesired constituent from a mixture of particulate matter, comprising: awall defining a flotation cell; an aerator for aerating a mixture ofparticulate matter to produce froth; a feed opening for introducing amixture of particulate matter and/or froth into said cell; a dischargeorifice in a wall of the cell; a froth support in communication withsaid discharge orifice for receiving and supporting said froth; and awash sprayer disposed upstream of said discharge orifice, wherein saidsupport has a continuous cross-sectional perimeter and comprises a vent.30. The apparatus of claim 29, wherein the vent size and/or the ventlocation is variable.
 31. The apparatus of claim 29, comprising a frothdepth controller.
 32. The apparatus of claim 29, wherein said frothsupport comprises a bottom interior surface of a conduit having anaverage width greater than an average height.
 33. The apparatus of claim29, further comprising a base having a major and wherein said frothsupport is disposed at an upwardly-inclined angle with respect to saidmajor plane of said base.
 34. The apparatus of claim 29, wherein atleast a portion of said froth support is disposed above said flotationcell.
 35. The apparatus of claim 29, wherein said froth support has aconvoluted path for fluid travel.
 36. The apparatus of claim 35, whereinsaid froth support comprises a first support section disposed at a firstupwardly-inclined angle with respect to said major plane of said base;and a second support section disposed at a second upwardly-inclinedangle, same or different, with respect to said major plane of said base,wherein said first and second sections are disposed in opposingdirections with respect to a plane perpendicular to said major plane ofsaid base.
 37. The apparatus of claim 29, further comprising a frothmotivator.
 38. The apparatus of claim 29, further comprising a screendisposed in proximity to and/or upstream of said discharge orifice in hepath of travel of at least a portion of the generated froth.
 39. A frothflotation apparatus for separating a desired constituent from a mixtureof particulate matter, comprising: a wall defining a flotation cell; anaerator for aerating a mixture of particulate matter to produce froth; afeed opening for introducing a mixture of particulate matter and/orfroth into said cell; a discharge orifice in a wall of the cell; a frothsupport in communication with said discharge orifice for receiving andsupporting said froth; a wash sprayer disposed upstream of saiddischarge orifice; and a froth motivator disposed in substantiallysealing relationship with a bottom portion of said support.
 40. Theapparatus of claim 39, comprising a froth depth controller.
 41. Theapparatus of claim 39, wherein said froth support comprises a bottominterior surface of a conduit having an average width greater than anaverage height.
 42. The apparatus of claim 39, further comprising a basehaving a major plane and wherein said froth support is disposed at anupwardly-inclined angle with respect to said major plane of said base.43. The apparatus of claim 39, wherein at least a portion of said frothsupport is disposed above said flotation cell.
 44. The apparatus ofclaim 39, wherein said froth support comprises a froth drainer tosubstantially separate wash fluid containing dislodged particles fromcontact with one or more of separated froth and washed-froth.
 45. Theapparatus of claim 39, further comprising a screen disposed in proximityto and/or upstream of said discharge orifice in the path of travel of atleast a portion of the generated froth.